sc7280: Increase SPI frequency to 50 MHz

Based on the datasheet, we can safely increase the SPI frequency of
sc7280 to 50 MHz.

BUG=b:190231148
BRANCH=None
TEST=build and boot BIOS with this config on herobrine boards

Change-Id: I84420d7d8ab0cb979fc606fcf05147197bc51c35
Signed-off-by: Shelley Chen <shchen@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/63948
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

src/soc/qualcomm/sc7280/bootblock.c

@@ -8,6 +8,6 @@
 void bootblock_soc_init(void)
 {
 	clock_init();
-	quadspi_init(37500 * KHz);
+	quadspi_init(50000 * KHz);
 	qupv3_fw_init();
 }