mb/google/bluey: Specify ROM size per board variant

This commit refines the ROM size configuration for Google Bluey
mainboards by moving the `BOARD_ROMSIZE_KB_*` selection from the
common Kconfig to the specific board variants.

Previously, `BOARD_ROMSIZE_KB_65536` was selected for
`BOARD_GOOGLE_BLUEY_COMMON`. This blanket setting is inaccurate as
different Bluey-based boards utilize different sized ROMs.

With this change:
- `BOARD_GOOGLE_BLUEY` now selects `BOARD_ROMSIZE_KB_65536`.
- `BOARD_GOOGLE_QUENBI` now selects `BOARD_ROMSIZE_KB_32768`.

This ensures that each board variant has the correct ROM size
configured, preventing potential issues related to image size or
flash programming.

TEST=Able to verify that google/bluey AP FW image os 64MB and
google/quenbi AP FW is 32MB in size.

Change-Id: Ifcf75b310f08cbc4bbbd27ad7947df06da4f5d5a
Signed-off-by: Subrata Banik <subratabanik@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/88268
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Pranava Y N <pranavayn@google.com>
Reviewed-by: Dinesh Gehlot <digehlot@google.com>
Reviewed-by: Kapil Porwal <kapilporwal@google.com>

src/mainboard/google/bluey/Kconfig

@@ -2,7 +2,6 @@
 
 config BOARD_GOOGLE_BLUEY_COMMON
 	def_bool n
-	select BOARD_ROMSIZE_KB_65536
 	select COMMON_CBFS_SPI_WRAPPER
 	select MAINBOARD_HAS_CHROMEOS
 	select SOC_QUALCOMM_X1P42100
@@ -15,12 +14,14 @@ config BOARD_GOOGLE_BASEBOARD_BLUEY
 
 config BOARD_GOOGLE_BLUEY
 	select BOARD_GOOGLE_BASEBOARD_BLUEY
+	select BOARD_ROMSIZE_KB_65536
 	select MAINBOARD_HAS_FINGERPRINT_VIA_USB
 	select MISSING_BOARD_RESET
 	select SPI_FLASH_MACRONIX
 
 config BOARD_GOOGLE_QUENBI
 	select BOARD_GOOGLE_BASEBOARD_BLUEY
+	select BOARD_ROMSIZE_KB_32768
 	select MAINBOARD_HAS_CHROME_EC
 	select MAINBOARD_HAS_FINGERPRINT_VIA_SPI
 	select MAINBOARD_HAS_GOOGLE_TPM